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On Fri, 13 Sep 2019 at 01:13, Wes Stewart via Groups.Io <n7ws=
[email protected]> wrote:

I'm confused. Are you saying that near zero length opens or shorts aren't
opens and shorts at low frequency and a 50 ohm resistor isn't 50 ohm?

Wes N7WS

Yes, I am if the short, open or load are on the end of a transmission line.

Everyone knows that the impedance of a short or open on the end of a
transmission line changes due to the physical length of the transmission
line. Stubs are often made to exploit that. so let’s ignore that
temporarily and consider only the load.

A perfect 50 ohm load on a piece of lossless coaxial cable would appear as
50 from DC up to the maximum frequency supported by the transmission line.

Now if the transmission line is not lossless, but has the typical losses of
a coaxial cable, then at low frequencies (say 1 MHz) that 50 ohm
transmission rises, so now the 50 ohms no longer appears as 50 ohms.

This effect is small, since for a typical calibration standard, the length
of transmission line is only 15 mm or less.

Since the load is always the poorest component in a good calibration kit,
now let’s consider the short calibration standard, or “offset short” to
give it s more appropriate name. We all know that the impedance of a short
on the end of a coax changes as the electrical length of the coax is
changed. We can predict that easily. If a shorted coax is a quarter wave
long, it appears as an open.

Now at a frequency of say 500 kHz, that coax is now no longer 50 ohms, but
might be a few hundred ohms. So the simple theoretical model that considers
on the electrical length, would no longer be valid.

This effect is going to be small on a coaxial calibration standard only has
a length of a few mm. The combination of the low frequency and the short
physical length means that the effect is small. But it certainly does
exist.


--
Dr. David Kirkby,
Kirkby Microwave Ltd,
drkirkby@...

Telephone 01621-680100./ +44 1621 680100

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